Welding of High Strength DMR Grade Steel

DEVELOPMENT OF SMAW ELECTRODE FOR WELDING OF HIGH STRENGTH DMR GRADE STEEL

Introduction

Previously, the Navy used to source special steel plates from overseas suppliers. Now Indian steel manufacturers have developed and produced low alloy steel plates having very high tensile properties with the help of DRDO for use in ship building applications. In association with the Indian Navy and DRDO, SAIL has developed DMR high strength grade steel.

The welding of this material with suitable consumable has become necessary. Therefore Indian Navy and NMRL have taken up a project to develop suitable consumables through Indian consumable manufacturers. Apart from developing consumables, understanding details pertaining to the welding procedural aspect like joint design, selection of suitable size of consumable, Inter-pass temperature, number of layers, heat input is essential as they can influence the properties of the weld metal.

This paper details the developmental work undertaken to develop electrodes for welding high strength DMR grade steel and also establish the parameters to get desired properties. The present steel is a nickel bearing micro alloyed steel characterized by higher strength and superior toughness even at sub zero temperatures as low as minus 50oC. These plates were hitherto being imported in quenched and tempered condition. This steel has got Cr, higher Ni, Cu and Mo also. Because of the presence of these elements it yields higher strength with good toughness at minus 50oC. The chemical and mechanical property of this steel is shown in Table 1. 

Basically, the alloy additions after the transformation characteristics of the steel enable to

achieving a higher strength and toughness after heat treatment. Fig.1 shows an isothermal diagram of typical Q&T steel. It can be observed from this diagram. The starting of the austenite to pearlite (or) ferrite transformation requires long durations and therefore even moderate / slow cooling produced martensite / lower bainite and avoid ferrite. Indian Navy in association with NMRL has specified the weld metal property requirements. The details of the various properties to be met by the weld metal are specified in the Table 2. 

With this understanding, the electrode development work was under taken.

Developmental Works

During development, several batches were produced, tested for all properties with boiler

quality mild steel plates before standardizing. During formulating the suitable chemistry, following aspects were also kept in mind.

(a) Lower levels of impurity elements in the weld metal.

(b) Selection of suitable binder to get extra low hydrogen in the weld metal.

(c) Effect of dilution with base material.

(d) Sufficient percentage of Mn & Ni to get desired structure.

Structural variation with thesealloy percentages are shown in Fig. 2. Thus an electrode meeting NMRL specification has been developed with the targeted chemistry as shown in Table 3.

Experimental Studies

For the purpose of this study, sufficient quantity of electrodes of size 3.15 and 4.0 mm was

produced and used for all internal laboratory tests and established the parameters with DMR plates. After qualifying the procedure at our R&D, it was sent for testing at NMRL, Ambernath. The design of groove is as shown in Fig. 3. The procedure established with the consumable is detailed in Table 4.

After satisfactory results were received from NMRL, few more batches were produced and

established the repeatability at NMRL & user end CSL, Cochin also. The results obtained at user end are shown in Table 5.

The effects of various parameters are discussed in the following paragraphs.

Results and Discussions

The results obtained through the above study were summarized as under:

Conclusions

From the results obtained we can conclude as follows.

(a) SMAW electrode meeting NMRL specification requirements for welding this steel has been developed.

(b) Lower IPT seems to have a beneficial effect especially when low temperature toughness

properties are desired.

(c) Lower heat input produced better toughness.

(d) A judicious choice of various welding parameters produces desired results.

(e) Proper root face advised to qualify the welders.

(f) Diffusible hydrogen levels control is very important to get good ductility.